1. Field of the Invention
The present invention relates to a material composition for a varistor and more particularly, to a material composition can be observed in a microstructure of a varistor wherein the material composition has a core-shell structure.
2. Description of the Related Art
In the electronic industry, the current trend is towards higher operational frequency and smaller volume. Thus, varistors or surge absorbers are conventionally used in high-frequency applications for protecting IC from getting damaged due to overvoltage.
Various materials, such as SrTiO3, SiC, ZnO, Fe2O3, SnO2, TiO2, BaTiO3 and Diode, may be implemented to absorb surge. However, since the materials present diverse characteristics in surge absorbent abilities, static absorbent abilities and other properties, not all of the mentioned materials can be applied in manufacture and practical applications.
A silicon diode provides surge absorbent ability relying on the PN interface. A varistor made from this material disadvantageously possesses relatively higher breakdown voltage and inferior surge absorbent ability.
Varistors constructed of Fe2O3 and BaTiO3 have the surge absorbent ability relying on the interface between electrodes and ceramics. However, such varistors present inferior electrical properties and are unsuitable for making high-voltage components.
The surge absorbent ability of varistors made from ZnO, TiO2, SnO2 or SrTiO3 typically depends on the interface between semi-conductive grains and grain-boundary insulating layers. However, the grain-boundary insulating layers are primarily composed of crystalline phases, such as crystalline phases of α-Bi2O3, Na2O or SrTiO3. Thus, the disadvantage of such varistors is that the production of the grain-boundary insulating layers requires sintering at a relatively higher temperature.